This document provides an overview of distillation columns and separation train design. It discusses different types of distillation columns including plate columns with bubble cap trays and sieve trays, and packed columns. It also covers concepts like equilibrium lines, operating lines, minimum reflux ratio, and McCabe-Thiele diagrams. The document discusses methods for determining the number of columns needed, possible sequences, and how to evaluate sequences using the marginal vapor rate method. Finally, it provides heuristics for sequencing separation points in a distillation train.

1. CH EN 5253
Process Design II
Lecture 09
Separations: Distillation Columns and Trains
January 27, 2020

2. Books
Product and Process Design Principles: Synthesis, Analysis and Evaluation
by J. D Seader, and Warren D. Seider and Daniel R. Lewin,
• Chapter 9
2

3. Distillation

4. Distillation Column Internal
4

5. Plate Types
Bubble Cap Tray Sieve Tray

6. Packed Towers
• Random Packing
• Structured Packing
Note: Importance of Distributor plate

7. Equilibrium Equation
• Relative Volatility
• Equilibrium Line
α=KL/KH

8. Operating and Feed Lines
• Rectifying Section
– R = reflux ratio
– V = vapor flow rate
• Stripping Section
– VB = Boil-up ratio
• Feed Line

9. Minimum Reflux Ratio

10. McCabe-Thiele

11. Step Off Equilibrium Trays

12. Short cut to Selecting a Column Design
• Minimum Cost for Distillation Column will
occur when you have a
– Minimum of Total Vapor Flow Rate for column
– Occurs at
• R = 1.2 Rmin @ N/Nmin= 2
• Nmin= log[(dLK/bLK)(bHK/dHK)]/log[αLK,HK]
• Rmin ≈ (F/D)/(α-1)
– V = D(R+1)
• V = Vapor Flow Rate
• D = Distillate Flow Rate (Production Rate)
• R = Reflux Ratio

13. How To Determine the Column Pressure
• Cooling Water Available at 90°F
• Distillate Can be cooled to 120°F min.
• Calculate the Bubble Pt. Pressure of Distillate
Composition at 120°F
– equals Distillate pressure
– Bottoms pressure = Distillate pressure + 10 psia ΔP
• Compute the Bubble Pt. Temp for an estimate of the
Bottoms Composition at Distillate Pressure
– Give Bottoms temperature
• Not Near Critical Point for mixture

14. Design Issues
• Packing vs Trays
• Column Diameter from flooding consideration
– Relations in Ch 17 of Towler (Design I)
• Column Height
– Relatiopns in Ch 17 of Towler
– N=Nmin/ε (or 2 Nmin/ ε)
• Column Height = N*Htray
• Tray Height = typically 1 ft (or larger)
• Packed Height = Neq*HETP (or 2 Neq*HETP)
– HETP(height equivalent of theoretical plate)
– HETPrandom = 1.5 ft/in*Dp Rule of thumb
• Tray Efficiency, ε = f(viscosityliquid * αLK,HK)
• Pressure Drop
• Tray, ΔP=ρLg hL-wier N
• Packed, ΔP=Packed bed (weeping)

15. Column Operating Window
• Plate design must ensure good contacting between phases
– Coning: vapor bypasses liquid
– Weeping: liquid drains through to tray below
• Usually design to operate near (~ 70 to 80% of) flooding limits
so as to allow for turn-down
Liquid rate
Vapor
rate
Jet flooding
Downcomer flooding
Weeping
Coning
Excessive
entrainment
Area of satisfactory
operation

16. Column Costs
• Column – Material of Construction gives ρmetal
– Pressure Vessel Cp= FMCv(W)+CPlatform
– Height may include the reboiler accumulator tank
– Tray Cost = N*Ctray(DT)
– Packing Cost = VpackingCpacking + Cdistributors
• Reboiler CB α AreaHX
• Condenser CB α AreaHX
• Pumping Costs – feed, reflux, reboiler
– Work = Q*ΔP
• Tanks
– Surge tank before column, reboiler accumulator, condensate accumulator
– Pressure Vessel Cp= FMCv(W)+CPlatform

17. Use of Separation Units

18. Reaction
Hydrodealkylation of Toluene
Toluene + H2 ⟷ Benzene + CH4
2 Benzene ⟷ Biphenyl + H2
Reactor Effluent
T = 1,350 °F
P = 500 psia
Example

19. Reactor Effluent
Component kmole/hr
Hydrogen 1292
Methane 1167
Benzene 280
Toluene 117
Biphenyl 3
Total 2859
Reaction Conditions
T = 1,350 °F
P = 500 psia

20. After Flash to 100F @ 500 psia
Effluent Vapor Liquid
Component kmole/hr kmole/hr kmole/hr
Hydrogen 1292 1290 2
Methane 1167 1149 18
Benzene 280 16 264
Toluene 117 2 115
Biphenyl 3 0 3
Total 2859 2457 402
Recycled Reactants

21. Further Separation
What separation units should be used?
• Liquid Separation
– Toluene, BP = 111°C
– Benzene, BP = 80°C
– What happens to the Methane (BP = –162 °C) and Biphenyl
(BP = 256°C) impurities?
• Gas Separation
– Hydrogen
– Methane
– What happens to the Toluene and Benzene impurities?

22. Direct Distillation Sequence

23. Column Sequences
• Number of Columns
– Nc = P – 1
• P = Number of Products
• No. of Possible Column Sequences
– Ns = [2(P–1)]! / [P!(P–1)!]
• P = Number of Products
P = 3, Nc = 2, Ns = 2
P = 4, Nc = 3, Ns = 5
P = 5, Nc = 4, Ns = 14
P = 6, Nc = 5, Ns = 42
P = 7, Nc = 6, Ns = 132
Number of possible
column sequences
becomes very large
very quickly!

24. Example
• P = Number of Products = 4 ( A, B, C, D)
• Number of Columns
– Nc = P – 1= 4-1=3
• No. of Possible Column Sequences
– Ns = [2(P–1)]! / [P!(P–1)!]
= [2(4–1)]! / [4!(4–1)!] )
= [2X3]! / [4!X3!]
= 6! / [4!X3!]
= 720 / [24X6]
= 720 / 144
= 5
24

25. Example
5 Possible Column Sequences
25

26. Example
How do I evaluate which is the best
sequence of separation columns?
( 1 Problem in HW #4)
26

27. Example
Marginal Vapor Rate Method
27

28. Marginal Vapor Rate
• Marginal Annualized Cost Marginal Vapor Rate
• Marginal Annualized Cost proportional to
– Reboiler Duty (Operating Cost)
– Reboiler Area (Capital Cost)
– Condenser Duty (Operating Cost)
– Condenser Area (Capital Cost)
– Diameter of Column (Capital Cost)
• Vapor Rate is proportional to all of the above
~

29. Selecting Multiple Column Separation Trains
• Minimum Cost for Separation Train will occur
when you have a minimum of Total Vapor
Flow Rate for all columns
R = 1.2 Rmin
V = D(R+1)
V = Vapor Flow Rate
D = Distillate Flow Rate
R = Recycle Ratio

30. After Quench to 100F @ 500 psia
Effluent Vapor Liquid
Component kmole/hr kmole/hr kmole/hr
Hydrogen 1292 1290 2
Methane 1167 1149 18
Benzene 280 16 264
Toluene 117 2 115
Biphenyl 3 0 3
Total 2859 2457 402
Recycled Reactants

31. Direct Sequence Indirect Sequence
Distillate Flow Distillate Flow Distillate Flow Distillate Flow
Liquid Column 1 Column 2 Column 1 Column 2
kmole/hr
Hydrogen 2 x x x
Methane 18 x x x
Benzene 264 x x x
Toluene 115 x x
Biphenyl 3
Total 402 284 115 399 284
Sequence Total 399 683
R assumed to be similar for all columns and R > 1
Simplified Marginal Vapor Flow Analysis
(First two columns only)
D=

32. Separation Train Heuristics
1. Remove thermally unstable, corrosive, or chemically reactive
components early in the sequence.
2. Remove final products one by one as distillates (the direct
sequence).
3. Sequence separation points to remove, early in the sequence, those
components of greatest molar percentage in the feed.
4. Sequence separation points in the order of decreasing relative
volatility so that the most difficult splits are made in the absence of
the other components.
5. Sequence separation points to leave last those separations that give
the highest-purity products.
6. Sequence separation points that favor near equimolar amounts of
distillate and bottoms in each column.